41 6 LECTURE XXVI. 



germination. At the uppermost end of the germinal stem, shaded black, is still to be 

 recognised the growing-point marked v in /; also the growing-point w of the primary 

 root. New growing-points have now arisen in addition at ^ ^ M^, &c., and have 

 already in part proceeded to the formation of new lateral shoots. Each of these 

 new growing-points has originated, during progressive growth, out of the primary 

 growing-point, v in /. The lateral growing-points k were first protruded in the axils 

 of the two cotyledons, c, II: the primary growing-point then grew farther, and 

 produced the leaves b, V , h", b'", &c., a new growing-point arising each time from 

 the axil of the leaf, as it was put forth from the growing-point [>/ ^', Sec). In the 

 course of the growth leaf-buds have already been put forth from these new growing- 

 points. The leaves and portions of stem also produced from the primary growing- 

 point w, have in the meantime however grown much more rapidly, and have already 

 entered into the second and third stages of growth : the leaves, b', V, b"', as well as the 

 parts of the shoot-axis belonging to them, are still found in the elongating condition, 

 as indicated by the light shading. The cotyledons c, and the leaf b, as well as the 

 parts of the shoot-axis belonging to them, which are not shaded at all, are on the 

 other hand already fully developed externally — i. e. they have attained their definitive 

 volume, and their permanent external form. These parts, however, are now in the 

 third phase of growth — i. e. their internal development is now being completed ; the 

 lignification of the vascular bundles, the development of sieve-tubes, the thickening 

 of the walls of the parenchyma cells, formation of stomata and development of cuticle 

 on the epidermis are being completed in these portions. All these histological 

 differentiations have already commenced in the parts of the plants indicated by the 

 shading ; but it is only after they have attained their definitive volume and permanent 

 form that the histological development also is completed. 



Let us now take into consideration in the same way the subterranean root-system 

 of our plant (Fig. 254, ///). It is to be noted that the lateral roots it/ it/' were 

 produced originally close behind the growing-point {w) of the primary root. Each of 

 these lateral roots was originally itself only a new growing-point, by the further 

 growth. of which the filiform lateral roots have been produced ; and the same relation 

 exists between the lateral roots of the second order and their mother-roots. Mean- 

 while, it need only here be mentioned by the way that the growing-points of the new 

 roots originate in the interior of the tissue of their mother-roots; while the new 

 growing-points of the shoots are superficial outgrowths of the primary growing- 

 point. 



In the roots also the growing-points are represented black in our figure ; but, it 

 will be seen, they are surrounded by an additional light zone, the root-cap, which 

 originates simultaneously with the first rudiment of the growing-point of a root. 

 Behind each of the growing-points of the root, which are shaded black, is a short 

 piece marked by lighter shading : this is the elongating portion of the root-fibre, and, 

 as is seen at once, it is strikingly short in comparison with the elongating region of 

 the shoot-axis. 



A very important point, which will already have been iioticed by the way in the 

 above description, must now be made quite clear. All the growing-points of the plant 

 ///, shaded black, originated in the first place from the two growing-points v and w 

 of the embryo /; and these themselves are both, as already said, simply remnants of 



